Gallium oxide (Ga2O3) is a transparent semiconductor which has a wide band gap of 4.8 to 5.3 eV at room temperature and absorbs almost no visible light or ultraviolet light. For this reason, gallium oxide is a promising material for use in optical and electronic devices and transparent electronics which operate in the deep-ultraviolet region. Gallium oxide (Ga2O3)-based photodetectors, light-emitting diodes (LEDs), and transistors is being developed in recent years [see Non-Patent Literature 1 (Jun Liang Zhao et al, “UV and Visible Electroluminescence From a Sn:Ga2O3/n+-Si Heterojunction by Metal-Organic Chemical Vapor Deposition”, IEEE Transactions on Electron Devices, VOL. 58, No. 5, May 2011)].
Gallium oxide (Ga2O3) has five crystal structures, α, β, γ, σ, and ε. Typically, the most stable structure is β-Ga2O3. However, as β-Ga2O3 has a β-galia structure, which is not a crystal structure commonly used as electronic materials or the like, the application of β-Ga2O3 to semiconductor devices is not always suitable. Further, growing a β-Ga2O3 thin film requires a high substrate temperature or high degree of vacuum, which would increase manufacturing costs. Furthermore, as described in Non-Patent Literature 2 (Kohei Sasaki et al, “Si-Ion Implantation Doping in β-Ga2O3 and Its Application to Fabrication of Low-Resistance Ohmic Contacts,” Applied Physics Express 6 (2013) 086502), annealing at a high temperature of 800 to 1100° C. is required after a dopant is ion-implanted into β-Ga2O3 in order to use the dopant as a donor even when a high concentration (e.g., 1×1019/cm3 or higher) of dopant (Si) is used.
On the other hand, α-Ga2O3 has the same crystal structure as sapphire substrates, which are already commercially available, and therefore is suitably used in optical and electronic devices. Currently, a gallium oxide thin film having electrical properties suitable for semiconductor devices has been desired.